Firefighters and fighter pilots’ lives depend on the temperature control systems built into the protective clothing that makes working in extreme conditions possible. When a developer of personal cooling systems needed an automated test stand to test their products in harsh environments, it turned to Viewpoint Systems (www.viewpointusa.com).
Aspen Thermal develops microclimate conditions systems, including miniature cooling systems that circulate water inside protective clothing and keep the body’s core temperature within safe limits during exposure to extreme temperatures.
Aspen Thermal’s miniature cooling system, based on a vapor compression cycle and a miniature rotary compressor, contains a small DC motor for variable operation of the compressor to meet the load demand. The miniature refrigeration unit chills a heat transfer liquid (usually water), which is circulated to a tube-lined garment worn by the user. A custom digital control system maintains the circulating liquid temperature at the user-specified setpoint by varying the compressor speed.
Range of conditions
Aspen Thermal needed an automated test stand for its personal cooling system fluid conditioning module (FCM) that would demonstrate the actual performance of each FCM at the rated load over a range of operating conditions. A fluid conditioning module must maintain the desired water output temperature—at multiple setpoints—regardless of the ambient temperature or the actual cooling load. The FCM test stand is designed to gather cooling performance, at the rated load, over a range of operating conditions.
A thermal test enclosure provides the stable, repeatable ambient temperature environment for the FCM. A liquid circulating loop (LCL) provides the variable cooling load for the test stand. Software proportional-integral-derivative (PID) control loops maintain the enclosure temperature at the desired ambient setpoint by controlling the pulse-width modulation (PWM) and maintaining actual heat load at the desired setpoint.
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Viewpoint Systems built a custom application using National Instruments’ LabView and its own toolkits to perform automated sequencing, control, measurement, analysis and reporting. The test application starts the LCL pump and heater, energizes the FCM condenser fan, turns on the FCM, and starts the PID controllers. A setpoint sequencer begins the test sequence, waits for each setpoint to stabilize, and gathers the test results. At the end of the sequence, the test application shuts down the PID controllers, the FCM, the LCL, and prints the final test report.
The application was developed and implemented by using NI LabView and a layered model view controller (MVC) architecture framework was tailored to Aspen’s specific test stand requirements. A tab-delimited text file drives the setpoint auto sequencer. Setpoint profiles can be quickly created, edited, and deployed to support changing test requirements or experimental testing.
Viewpoint Systems’ dBaser database toolkit reads test stand configuration data from a Microsoft Excel worksheet, directly into native LabView data types. This file contains all the specific information while allowing for a high degree of application flexibility.
Complete snapshots of all test results are time-stamped and written to the Excel results file using the dBaser toolkit. The final hardcopy test report with a graph is created by Excel and shipped with each completed FCM.
When the client decided to switch from thermocouples to resistance-temperature detector (RTD) sensors, the conversion was quickly implemented: change the sensors, swap hardware I/O modules, and reconfigure the device and channel model parameters in the Excel worksheet. No code changes were required.
Aspen is now able to test a completed FCM automatically under all desired operating conditions to ensure compliance with performance specifications. The MVC architecture provides the necessary performance, safety and flexibility required for demanding test applications.
Jim Campbell, [email protected], is president of Viewpoint Systems, which specializes in automation and control of measurements and is a member of the Control System Integrators Association.
